title: Aromatic interactions in asymmetric catalysis: control of enantioselectivity in Diels-Alder reactions catalysed by camphor-derived hydrazides.
abstract: Density functional theory calculations (M06-2X//B3LYP) have been performed to determine the factors responsible for enantioselectivity in Diels–Alder reactions catalysed by two series of camphor-derived amines. Hydrazides 2 and sulfonylhydrazides 3 catalyze the reaction of cyclopentadiene with cinnamaldehyde to give the same enantiomer of cycloadduct. The calculations reveal that the two classes of catalysts control enantioselectivity by opposite mechanisms. Hydrazides 2 favour addition to the bottom face of a trans iminium cation, while sulfonylhydrazides 3 favour addition to the top face of a cis iminium ion. In the transition state for cycloadditions catalysed by 2, a stabilising CH–π interaction between the diene and a benzyl substituent α to the iminium nitrogen accelerates the reaction and enhances the enantioselectivity. The facial selectivity can be reinforced by appending onto the benzyl side-arm an α-methyl group that sterically hinders addition to the top face.
authors.affiliation: School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia. e.krenske@uq.edu.au
authors.forename: Elizabeth H
authors.initials: EH
authors.lastname: Krenske
mesh_terms: D000588:Amines / Q000737:chemistry; D002164:Camphor / Q000737:chemistry*; D002384:Catalysis; D061565:Cycloaddition Reaction; D006834:Hydrazines / Q000737:chemistry*; D008958:Models, Molecular; D013237:Stereoisomerism
chemical_list: D000588:Amines; D006834:Hydrazines; D002164:Camphor